Cloud Messaging registration fails on Android 14/15/16

After a clean installation of GmsCore (version v0.3.x or newer), the device fails to register for Google Cloud Messaging (GCM). While the "Check-in" service appears to be working correctly, apps that rely on push notifications (e.g., Signal, WhatsApp, YouTube) do not appear in the Cloud Messaging list, or stay in a "Connecting" state indefinitely.

Steps to Reproduce
Install microG GmsCore and GsfProxy.

Enable "Google Check-in" and "Google Cloud Messaging" in microG Settings.

Restart the device.

Open an app that requires push notifications.

Check microG Settings -> Cloud Messaging. The app is either missing or status is "Disconnected".

Expected Behavior
Apps should register automatically with GCM, and notifications should be delivered promptly.

Environment
microG GmsCore version: [0.3.1.240913]

Android version: [14]

ROM: [Es. LineageOS 22l]

Device: [Galaxy Note 8]

Start by reproducing the failure with GmsCore v0.3.1.240913 and GsfProxy on Android 14, following the listed Check-in and Cloud Messaging steps. Investigate why apps are missing from the Cloud Messaging list or remain disconnected; done means apps register automatically and notifications are delivered promptly.

Written by the indexing model from the issue text.